In recent years, liquid crystal display (LCD) devices, which are rapidly replacing cathode ray tubes (CRTs), have been widely used in liquid crystal televisions, monitors, mobile phones, and the like by taking advantage of their features such as energy saving, a small thickness, and light weight. These features have been more effectively used in improvement of illumination devices that are positioned behind the LCD devices (so-called backlights).
The illumination devices are roughly divided into a side light type (which is also called an “edge light type”), and a direct type. In the side light type illumination devices, a light guide body is provided behind an LCD panel, and a light source is provided at a lateral end of the light guide body. Light emitted from the light source is reflected by the light guide body to be indirectly and uniformly applied to the LCD panel. This structure can reduce the thickness, and also can implement illumination devices having high uniformity of the brightness. Thus, the side light type illumination devices have been used primarily in small and medium sized LCDs, such as mobile phones and notebook computers.
An example of the side light type illumination devices includes an illumination device described in Patent Document 1. Patent Document 1 describes a surface-emitting device in which a plurality of dots are formed on a reflective surface of a light guide plate so that light can be uniformly emitted from a light-emitting surface. In this surface-emitting device, the corners of the reflective surface have a higher dot density than that of other part, because no light is transmitted to the corners due to directivity of a light source, and the corners become dark.
Moreover, in the direct type illumination devices, a plurality of light sources are arranged behind an LCD panel to apply light directly to the LCD panel. Thus, high brightness tend to be obtained even in large screen displays, and the direct type illumination devices have been used primarily in large LCDs of 20 inches or more. However, current direct type illumination devices have a thickness as large as about 20 to 40 mm, which hinders further reduction in thickness of displays.
It is possible to achieve further reduction in thickness of large LCDs by reducing the distance between the light sources and the LCD panel. In this case, however, uniformity of the brightness of the illumination devices cannot be obtained unless the number of light sources is increased. On the other hand, increasing the number of light sources increases the cost. Thus, it has been desired to develop thin illumination devices having high uniformity of the brightness, without increasing the number of light sources.
In view of the above problem, attempts have been made to reduce the thickness of large LCDs by arranging a plurality of side light type light-emitting elements.
As such a light-emitting element, Patent Document 2, for example, discloses a tandem type surface light source device having a tandem structure in which plate-like light guide blocks are arranged in tandem, and primary light sources for supplying primary light to the respective light guide blocks are provided. Patent Document 2 describes that this surface light source device can ensure a wide light emission area by using a compact structure.
Moreover, Patent Document 3 discloses an illumination device having collimating means provided on incident portions of light guide bodies. Patent Document 3 describes that this illumination device can increase the efficiency of incidence on the light guide bodies.
Moreover, Patent Document 4 discloses an illumination device in a form that adjusts light propagation by the difference in density of openings and reflecting portions.
Patent Document 4 describes that this illumination device can reduce the weight of illumination devices, and can implement uniform in-plane brightness distribution of illuminating light.
Moreover, Patent Document 5 discloses an illumination device in a form in which a light-emitting surface of a light guide body of an edge type backlight unit has a concavo-convex shape. Patent Document 5 describes that this structure enables illumination devices to be manufactured inexpensively by a simple method.
Moreover, Patent Document 6 discloses an edge type backlight in which light sources are light-emitting diodes (LEDs), and ridges are formed on a light guide plate. Patent Document 6 describes that, by changing an angle of the prism shape between a prism positioned in front of each LED and a prism positioned between LEDs, light can be directed also to the gap between the LEDs, and dark portions can be improved.    Patent Document 1: Japanese Published Patent Application No. 2003-43266    Patent Document 2: Japanese Published Patent Application No. H11-288611    Patent Document 3: Japanese Published Patent Application No. 2001-42328    Patent Document 4: Japanese Published Patent Application No. H08-297282    Patent Document 5: Japanese Published Patent Application No. 1-107-333442    Patent Document 6: Japanese Published Patent Application No. 2007-123086